Some Escherichia coli carry plasmids which confer on the host bacterium the ability: 1) to produce antibiotic-like protein, called colicin whose adsorption to specific bacterial receptors leads to cell death, and 2) to survive treatment with the homologous colicin. To further our understanding of colicin action, we have undertaken a program 1) to study the events associated with the induced synthesis of colicin, 2) to determine the molecular details underlying the mechanism whereby colicin is able to depolarize the bacterial energy transducing membrane, 3) to elucidate the mechanism of colicin immunity, and 4) to delineate the molecular interactions involved in the regulation of colicin receptor biosynthesis by iron.